China's power grid has become more and more complex. It has developed into an expansive, complicated system with various types of power sources and loads, and power transmission and transformation equipment having different voltage levels. In recent years, with the increasing requirements of environmental protection and resource conservation, the investment and cost for expanding the scale of the power grids and adding new transmission lines has become high. Therefore, how to make full use of existing power generation resources and power transmission and transformation lines has become an increasingly important issue of the power grid.
The flexible alternating current transmission system (FACTS) technology can greatly improve the power transmission capacity and the controllability of load flow and voltage without changing the structure of the power grid, thereby providing an effective solution to the difficulties existing in the operation and development of the power grid. The unified power flow controller (UPFC), considered to be the most powerful flexible power transmission device, can separately adjust the active and reactive power and node voltage of the lines, simultaneously, can execute the function of load flow control and increase the power transmission limit of the sections, and provides voltage support, thus having a wide application.
Currently, most of the research on UPFC are based on traditional UPFC topology. However, a UPFC device with a new topology has appeared in practice. Taking Chinese UPFC demonstration project, i.e. the West Network of Nanjing, which commenced operation in the year of 2015, as an example, the topology of its UPFC device is different from that of the traditional UPFC. Specifically, it includes three modular multilevel converters. A shunt converter is connected to the 35 kV bus of a 220 kV Yanziji main transformer, and the other two series converters are respectively connected to two parallel running 220 kV lines between the Tiebei station and the Xiaozbuang station through the series transformer. The UPFC with new topology structure reduces the size and cost of the shunt converter and improves the control of section load flow while the reactive power demands of the system is still satisfied.
The calculation of the load flow for power systems with the similar new UPFC topology is an essential part of pertinent engineering design. However, the existing load flow calculation methods seldom take this UPFC topology into consideration, thus there are still some difficulties in calculating such systems.